A magnetic reed switch is used to detect the opening and closing of the door. The switch consists of two metal contacts that close or open in the presence of a magnetic field (provided by a magnet).
NPN Transistor (BC547):
The NPN transistor is used as a switch. When the magnetic reed switch changes its state, the transistor is biased accordingly.
Resistor (R1):
A resistor is connected between the base of the NPN transistor and the positive voltage rail. This resistor is used for biasing the transistor.
Buzzer or Alarm:
A buzzer or alarm is connected to the collector of the NPN transistor. When the door is opened (magnetic reed switch changes state), the transistor turns on, allowing current to flow to the alarm.
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